Crushable spacer and bolted joint for a gas turbine engine

ABSTRACT

A bolted joint apparatus includes: a first component including a first row of first bolt holes extending therethrough; a second component including a second row of second bolt holes extending therethrough wherein the second bolt holes are aligned coaxially with the first bolt holes, a plurality of fasteners, each of fasteners disposed through aligned pairs of the first and second bolt holes to couple together the first and second components, each of the fasteners including a shank; and crushable spacers disposed around the shanks of the fasteners, the crushable spacers clamped in compression between the fasteners and one of the components, wherein each of the crushable spacers has a tubular body interconnecting first and second enlarged ends, the tubular body being defined by a peripheral wall which incorporates at least one weakening feature.

BACKGROUND OF THE INVENTION

This invention relates generally to bolted joints, and more specificallyto bolted joints within gas turbine engines.

A gas turbine engine includes a turbomachinery core having ahigh-pressure compressor, combustor, and high pressure turbine in serialflow relationship. The core is operable in a known manner to generate aprimary flow of propulsive gas. A typical turbofan engine adds alow-pressure turbine driven by the core exhaust gases which in turndrives a fan rotor through a shaft to generate a bypass flow ofpropulsive gas. In the case of a high bypass engine this provides themajority of the total engine thrust.

Bolted joints are used in several locations in a gas turbine engine.Each bolted joint includes abutting flanges which are held together bybolts or other fasteners through bolt holes provided in each of theflanges that are aligned with each other. During operation, the enginecan experience unusual events such as the fracture and release of a fanblade (“FBO”). This can produce extreme stresses, forces, and deflectionall over the engine. Large radial, tangential, or axial loads withrespect to an axial centerline of the bolted joint can impose bendingmoments or tensile forces in the flange that can cause deformation orrupture of the bolts. It is an FAA certification requirement that duringsuch an event the engine must retain its structural integrity, so thatcomponents do not strike other portions of an aircraft, or fall off ofthe aircraft.

In order to mitigate the forces on the engine, there are severallocations within the engine which use “fused” connections. One type offuse element is a crushable spacer, which is a small generallycylindrical device placed over the shank of a bolt and clamped togetherin a bolted joint. The spacer is able to resist the bolted joint preloadwhile retaining structural integrity, but if an excessive load isincurred, the spacer will crush/fail thus relieving stress in adjoiningcomponents. Under the circumstances, if the crushable spacers fail theconnected components will generally remain connected sufficiently toensure a safe outcome, but if they retain their integrity they maytransfer extreme forces to the surrounding components causing them tofail in unpredictable ways.

In the prior art, the crushable spacer is sized by selecting its wallthickness and material, which is typically a metal alloy. One problemwith bolted joints using existing spacers is that is that theforce/deflection characteristics are undesirable. Specifically, there isa region of elastic deflection, followed by yielding, followed byeventual tensile failure. In operation, there can be a large differencein the load required to produce elastic deflection and the load requiredto produce failure. The crushable spacer can experience a very largedegree of yielding before spacer column buckling occurs. The net effectis that the spacer fails in a drawn-out fashion and that a plurality ofspacers subjected to an overload can often fail inconsistently atdifferent times. This defeats the purpose of the crushable spacer.

BRIEF DESCRIPTION OF THE INVENTION

This problem is addressed by a bolted joint incorporating a crushablespacer having a peripheral wall incorporating at least one weakeningfeature.

According to one aspect of the technology described herein, a boltedjoint apparatus includes: a first component having a first flangeincluding a first row of first bolt holes extending therethrough; asecond component having a second flange including a second row of secondbolt holes extending therethrough wherein the second bolt holes arealigned coaxially with the first bolt holes, a plurality of fasteners,each of the fasteners disposed through aligned pairs of the first andsecond bolt holes to couple together the first and second flanges, eachof the fasteners including a shank; and crushable spacers disposedaround the shanks of the fasteners, the crushable spacers clamped incompression between the fasteners and one of the flanges, wherein eachof the crushable spacers has a tubular body including a central portioninterconnecting first and second enlarged ends, the tubular body beingdefined by a peripheral wall which incorporates at least one weakeningfeature.

According to another aspect of the technology described herein, a gasturbine engine includes: turbomachinery components including a fan, acompressor, a compressor, a combustor, and a turbine arranged in serialflow relationship and circumscribed about an engine centerline axis; anda supporting static structure circumscribing the turbomachinerycomponents; wherein at least one of the turbomachinery components andthe static structure include a bolted flange assembly, including: afirst component having a first flange including a first row of firstbolt holes extending therethrough; a second component having a secondflange including a second row of second bolt holes extendingtherethrough, wherein the second bolt holes are aligned coaxially withthe first bolt holes; a plurality of fasteners, each of the fastenersdisposed through aligned pairs of the first and second bolt holes tocouple together the first and second flanges, each of the fastenersincluding a shank; and crushable spacers disposed around the shanks ofthe fasteners, the crushable spacers clamped in compression between thefasteners and one of the flanges, wherein each of the crushable spacershas a tubular body including a central portion interconnecting first andsecond enlarged ends, the tubular body being defined by a peripheralwall which incorporates at least one weakening feature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is a schematic half-sectional view of a gas turbine engine;

FIG. 2 is a schematic cross-sectional view of a bolted joint of theengine of FIG. 1;

FIG. 3 is a side elevation view of a crushable spacer of the boltedjoint;

FIG. 4 is an end view of the crushable spacer of FIG. 3;

FIG. 5. is a cross-sectional view of the crushable spacer of FIG. 3;

FIG. 6 is a side elevation view of an alternative crushable spacer;

FIG. 7 is an end view of the crushable spacer of FIG. 7;

FIG. 8 is a cross-sectional view of the crushable spacer of FIG. 7;

FIG. 9 is a side elevation view of another alternative crushable spacer;

FIG. 10 is an end view of the crushable spacer of FIG. 9;

FIG. 11 is a cross-sectional view of the crushable spacer of FIG. 10;

FIG. 12 is a side elevation view of another alternative crushablespacer;

FIG. 13 is an end view of the crushable spacer of FIG. 12; and

FIG. 14 is a cross-sectional view of the crushable spacer of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 depicts anexemplary gas turbine engine 10. While the illustrated example is ahigh-bypass turbofan engine, the principles of the present invention arealso applicable to other types of engines, such as low-bypass turbofans,turbojets, turboprops, etc. The engine 10 has a longitudinal center lineor axis 11 and an outer stationary annular core casing disposedconcentrically about and coaxially along the axis 11.

It is noted that, as used herein, the terms “axial” and “longitudinal”both refer to a direction parallel to the centerline axis 11, while“radial” refers to a direction perpendicular to the axial direction, and“tangential” or “circumferential” refers to a direction mutuallyperpendicular to the axial and radial directions. As used herein, theterms “forward” or “front” refer to a location relatively upstream in anair flow passing through or around a component, and the terms “aft” or“rear” refer to a location relatively downstream in an air flow passingthrough or around a component. The direction of this flow is shown bythe arrow “F” in FIG. 1. These directional terms are used merely forconvenience in description and do not require a particular orientationof the structures described thereby.

The engine 10 has a fan 14, booster 16, compressor 18, combustor 20,high pressure turbine 22, and low pressure turbine 24 arranged in serialflow relationship. In operation, pressurized air from the compressor 18is mixed with fuel in the combustor 20 and ignited, thereby generatingcombustion gases. Some work is extracted from these gases by thehigh-pressure turbine 22 which drives the compressor 18 via an outershaft 26. The combustion gases then flow into the low-pressure turbine24, which drives the fan 14 and booster 16 via an inner shaft 28. Theengine 10 incorporates numerous bolted joints at several locations.Examples of potential bolted joints are in the circled areas in FIG. 1,labeled “B”.

Some non-limiting examples of locations where fused bolted joints wouldbe used are as follows: In a fan load reduction device (“LRD”); In a“backbone” joint between two annular portions of an engine casing (e.g.compressor case to combustor case); In a joint between a composite andmetal components, for example between a fan case and a fan inlet; Infuture applications, it is desirable to use this type of joint and otherlocations such as between a fan hub frame and a core cowl, where one ofthe components may be composite such as ceramic matrix composite orpolymeric matrix composite.

FIG. 2 illustrates a bolted joint 30 which is generally representativeof any of the bolted joints described above. A first component 32 (aportion of which is shown) includes a first flange 34. A secondcomponent 36 (a portion of which is shown) includes a second flange 38.In the illustrated example, each of the first and second components 32,36 is a body of revolution about the centerline axis 11; however, thisneed not be the case.

The first flange 34 incorporates at least one first bolt hole 40; forexample, a ring or row 42 of first bolt holes 40 may be provided. Thesecond flange 38 includes at least one second bolt hole 44; for example,a ring or row 46 of second bolt holes 44 may be provided. The firstflange 34 abuts the second flange 38 such that the first bolt holes 40are aligned coaxially with the second bolt holes 44.

Bolts 48 are disposed through the first bolt holes 40 and the secondbolt holes 44. Each of the bolts 48 includes a bolt head 50, a shank 52,and a thread 54.

Crushable spacers 56, described in more detail below, are disposedaround the shanks 50 of the bolts 48. The crushable spacers 56 contactand axially extend between the bolt heads 50 and a first annular surface58, preferably flat, of the first flange 34. Nuts 60 are provided havinginternal threads which engage the threads 54 of the bolt 48 and aretightened to provide a desired clamping load on the bolted joint 30,clamping the crushable spacers 56 in compression. It will be understoodthat the crushable spacers 56 could alternatively be placed on the otherside of the bolted joint 30, that is, between the second flange 38 andthe nuts 60.

It will be understood that the bolts 48 could be replaced with anyfastener capable of clamping together the first flange 34, the secondflange 38, and the crushable spacer 56 with a predetermined clampingload.

For example, instead of using the bolts 48 with nuts 60, one of theflanges 34, 38 could be provided with internal threads (not shown) toengage the threads 54 of the bolt 48. These could be formed, for exampleby a thread-cutting operation or by the installation of a threadedinsert into one of the flanges 34, 38.

As another example, instead of using the illustrated bolts 48 and nuts60, a threaded stud with nuts secured to both ends (not shown) could beused.

As yet another example, instead of using the illustrated bolts 48 andnuts 60, a fastener such as a conventional rivet or blind rivet (notshown) could be used.

FIGS. 3-5 show one of the crushable spacers 56 in more detail. Thecrushable spacer 56 has a body including a central portion 62 extendingaxially between and interconnecting first and second enlarged or flangedends 64, 66. The first and second ends 64, 66, define first and secondend faces 68, 70 respectively, which may be planar.

The crushable spacer 56 has a peripheral wall 72 defining a hollowinterior or through-bore 74. The body of the crushable spacer 56 maythus be generally described as a “tubular” form. In the illustratedexample the peripheral wall 72 is cylindrical or a body of revolution.Other cross-sectional shapes, such as regular or irregular polygons,could be used as well.

The peripheral wall 72 has a basic thickness “T” selected to haveparticular strength characteristics as described in more detail below.The peripheral wall 72 further includes at least one weakening feature.As used herein, the term “weakening feature” refers to a feature whichreduces the buckling strength or critical load of the crushable space56, as compared to a cylindrical wall of uniform basic thickness T.

The peripheral wall 72 includes a weakening feature embodied as at leastone opening formed therein. This opening may take numerous forms. In theexample illustrated in FIGS. 3-5, the opening takes the form of one ormore elongated slots 76 passing completely through the thickness of theperipheral wall 72 and having parallel sides 78 joined by rounded ends80.

The characteristics of the openings, including their number, shape,location, and dimensions, may be varied to suit a particularapplication. For example, the openings could take the form of singularor plural circular holes, or spiral slots (not shown). Optionally, theopenings could be “blind” openings which do not pass all the way throughthe thickness of the peripheral wall 72. In general, it is preferablethat the cross-sectional area remaining in the peripheral wall 72 besuitable to withstand the desired compressive force due to bolt preloadforces as well as expected forces during normal operation, but that thebuckling strength be reduced.

The material selected for the crushable spacer 56 should have agenerally high tensile strength and a ratio of ultimate tensile strength(“UTS”) to yield strength (“YS”) strength as low as possible. Thecrushable spacer 56 should be sized for yield strength YS at assemblyclamp load, and the buckling is based on an ultimate tensile strengthUTS for the load expected at an event such as blade out. The lower theratio of UTS/YS, the lower the flange load (past flange separation)needed to crush the crushable spacer 56 in a manner such that rapidpredictable failure will occur when loads exceed a predetermined stress,without a significant amount of yielding. The presence of a weakeningfeature as described herein further contributes to buckling and rapidpredictable failure.

The openings described above are one example of a weakening feature. thespecific mechanical design of the crushable spacer 56 may be varied tosuit a particular application, and different types of weakening featuresmay be implemented.

For example, FIGS. 6-8 illustrate an alternative crushable spacer 156similar in construction to the crushable spacer 56 described above. Thecrushable spacer 156 has a body including a central portion 162extending axially between and interconnecting first and second enlargedor flanged ends 164 and 166. The crushable spacer 156 has a peripheralwall 172 defining a hollow interior or through-bore 174. As noted above,this may be generally described as a “tubular” form. In the illustratedexample the peripheral wall 172 is cylindrical or body of revolution.Other cross-sectional shapes, such as regular or irregular polygons,could be used as well.

The peripheral wall 172 has a basic thickness “T” selected as describedabove. The peripheral wall 172 incorporates an “hourglass shape” whereinthe wall pinches inward such that an outer diameter “D1” near the centerof the peripheral wall 172 is less than a diameter “D2” near the flangedends 164, 166. This hourglass shape contributes to a reduced bucklingstrength as described above and may be considered a weakening feature.In addition to the hourglass shape described above, the crushable spacer156 may incorporate one or more openings as described above.

FIGS. 9-11 illustrate another alternative crushable spacer 256. Thecrushable spacer 256 has a body including a central portion 262extending axially between and interconnecting first and second enlargedor flanged ends 264 and 266. The crushable spacer 256 has a peripheralwall 272 defining a hollow interior or through-bore 274. As noted above,this may be generally described as a “tubular” form. In the illustratedexample the peripheral wall 272 is cylindrical or body of revolution.Other cross-sectional shapes, such as regular or irregular polygons,could be used as well.

The peripheral wall 272 has a basic thickness “T” selected as describedabove. The peripheral wall 272 incorporates at least one “kink” 276,defined herein as a discontinuity in the axial direction. In theillustrated example, the kink 276 takes the form of a generallysemicircular annular bulge located near the center of the peripheralwall 272. The presence of the kink 276 contributes to a low bucklingstrength as described above. Optionally, in addition to the hourglassshape described above, the crushable spacer 256 may incorporate at leastone opening as described above (not shown)

FIGS. 12-14 illustrate another alternative crushable spacer 356. Thecrushable spacer 356 has a body including a central portion 362extending axially between and interconnecting first and second enlargedor flanged ends 364 and 366. The crushable spacer 356 has a peripheralwall 372 defining a hollow interior or through-bore 374. As noted above,this may be generally described as a “tubular” form. In the illustratedexample the peripheral wall 372 is cylindrical or body of revolution.Other cross-sectional shapes, such as regular or irregular polygons,could be used as well.

The peripheral wall 372 has a basic thickness “T” selected as describedabove. The peripheral wall 372 incorporates at least one discrete thinsection 376 having a thickness “T1” which is less than the basicthickness T. The presence of the discrete thin section 376 contributesto a low buckling strength as described above. In the illustratedexample the thin section 376 is configured as an annular ring, but otherorientations such as axial or oblique are possible as well. Optionally,in addition to the thin section described above, the crushable spacer356 may incorporate at least one opening as described above (not shown).

The foregoing has described a crushable spacer and a bolted joint usinga crushable spacer for a gas turbine engine. All of the featuresdisclosed in this specification (including any accompanying claims,abstract and drawings), and/or all of the steps of any method or processso disclosed, may be combined in any combination, except combinationswhere at least some of such features and/or steps are mutuallyexclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying potential points of novelty, abstract and drawings), orto any novel one, or any novel combination, of the steps of any methodor process so disclosed.

What is claimed is:
 1. A bolted joint apparatus, comprising: a first component including a first row of first bolt holes extending therethrough; a second component including a second row of second bolt holes extending therethrough, wherein the second bolt holes are aligned coaxially with the first bolt holes, a plurality of fasteners, each of fasteners disposed through aligned pairs of the first and second bolt holes to couple together the first and second components, each of the fasteners including a shank; and crushable spacers disposed around the shanks of the fasteners, the crushable spacers clamped in compression between the fasteners and one of the components, wherein each of the crushable spacers has a tubular body including a central portion interconnecting first and second enlarged ends, the tubular body being defined by a peripheral wall which incorporates at least one weakening feature.
 2. The apparatus of claim 1 wherein the fasteners comprise bolts, each of the bolts including a bolt head, a thread, and a shank therebetween.
 3. The apparatus of claim 2 further comprising a plurality of nuts, each nut engaging the threads of one of the bolts.
 4. The apparatus of claim 1 wherein the at least one weakening feature is an opening formed in the peripheral wall.
 5. The apparatus of claim 4 wherein the opening passes completely through the peripheral wall.
 6. The apparatus of claim 4 wherein the opening is an elongated slot.
 7. The apparatus of claim 1 wherein the at least one weakening feature is an hourglass shape of the peripheral wall.
 8. The apparatus of claim 1 wherein the at least one weakening feature is a kink in the peripheral wall.
 9. The apparatus of claim 1 wherein the at least one weakening feature is a discrete thin section in the peripheral wall.
 10. The apparatus of claim 9 wherein the thin section is configured as an annular band.
 11. A gas turbine engine, comprising: turbomachinery components including a fan, a compressor, a compressor, a combustor, and a turbine arranged in serial flow relationship and circumscribed about an engine centerline axis; and a supporting static structure circumscribing the turbomachinery components; wherein at least one of the turbomachinery components and the static structure include a bolted flange assembly, including: a first component having a first flange including a first row of first bolt holes extending therethrough; a second component having a second flange including a second row of second bolt holes extending therethrough, wherein the second bolt holes are aligned coaxially with the first bolt holes; a plurality of fasteners, each of fasteners disposed through aligned pairs of the first and second bolt holes to couple together the first and second flanges, each of the fasteners including a shank; and crushable spacers disposed around the shanks of the fasteners, the crushable spacers clamped in compression between the fasteners and one of the flanges, wherein each of the crushable spacers includes a tubular body extending axially between first and second enlarged ends, the tubular body being defined by a peripheral wall that incorporates at least one weakening feature.
 12. The engine of claim 11 wherein the fasteners comprise bolts, each of the bolts including a bolt head, a thread, and a shank therebetween.
 13. The engine of claim 12 further comprising a plurality of nuts, each nut engaging the threads of one of the bolts.
 14. The engine of claim 11 wherein the at least one weakening feature is an opening formed in the peripheral wall.
 15. The engine of claim 11 wherein the opening passes completely through the peripheral wall.
 16. The engine of claim 14 wherein the opening is an elongated slot.
 17. The engine of claim 11 wherein the at least one weakening feature is an hourglass shape of the peripheral wall.
 18. The engine of claim 11 wherein the at least one weakening feature is a kink in the peripheral wall.
 19. The engine of claim 11 wherein the at least one weakening feature is a discrete thin section in the peripheral wall.
 20. The engine of claim 11 wherein the thin section is configured as an annular band. 